MILAN, Jan 23 (Reuters) - Milan prosecutors have opened a new investigation against Silvio Berlusconi over accusations he corrupted witnesses in a trial in which he was convicted of paying for sex with a minor.

The decision to open an investigation, announced on Thursday, adds to a long series of legal problems hanging over the leader of Italy's main centre-right party, who has been banned from parliament over a separate tax fraud conviction.

It may also complicate efforts to reform a complicated electoral system blamed for chronic instability in Italy, following an agreement at the weekend between Berlusconi's Forza Italia party and the centre-left Democratic Party.

Berlusconi was convicted last year of paying for sex with former nightclub dancer Karima El Mahroug, better known by the stage name "Ruby the Heartstealer", when she was under 18 and of abusing his powers as then-prime minister to get her released from police custody over unrelated theft accusations.

The 77-year-old media billionaire has appealed against the sentence, which will not take effect until the whole appeals process is completed.

In a statement on Thursday, Berlusconi repeated his accusations that judges and prosecutors had targeted him for political reasons and said he would not be deterred.

"I will remain in place, more convinced than ever of the need to keep fighting for what I believe in profoundly."

The latest investigation follows the conviction of three associates in a related trial in which they were found guilty of procuring young women for prostitution at parties at Berlusconi's luxurious villa near Milan.

Last year, judges in the trial requested prosecutors to consider opening an investigation into allegations that Berlusconi and his lawyers had met some of the young women for meetings to discuss the evidence they planned to give in court.

In a written judgment on the case, the court said there was evidence Berlusconi had paid "money and other benefits" to young women who had taken part in the parties to give misleading evidence in the case.

The investigation also targets Berlusconi's lawyers Niccolo Ghedini and Piero Longo, both of whom have denied accusations of witness tampering, as well as 42 other individuals. (Reporting by Manuela D'Alessandro; Writing by James Mackenzie; Editing by Robin Pomeroy)